During cataract surgery (and other lens surgery) the ophthalmic surgeon extracts the biological crystalline lens and replaces it with an artificial intraocular lens (IOL). If the optical properties of the artificial implant match the optical properties and the dimensions of the eye, the patient has a good chance that the vision after surgery can be good without needing spectacles, irrespective of the need for spectacles before surgery. FIG. 20 shows a schematic diagram of the human eye wherein the various anatomical parts and structures are indicated. FIG. 1 shows a simple model of the human eye. The refraction of light through the eye takes place in 1) the cornea and 2) the lens in order to focus the light at 3) the retina at the back of the eye. If there is an imbalance between any of the ocular components, the eye will need a spectacle correction to see clearly. FIG. 2 shows the phakic eye (top half) and the pseudophakic eye (bottom half). In the phakic eye the natural crystalline lens 22 is present. In the pseudophakic eye the natural crystalline lens has been substituted with a synthetic lens 23, typically an IOL implant. In the aphakic eye no lens is present.
Cataract is one of the leading causes of blindness and the surgical treatment of cataract is one of the most commonly performed surgical procedures world-wide. In recent years the overall improvement in safety and efficacy of lens surgery and the development of a number of new IOL designs have broadened the indication for lens surgery to encompass not only patients with cataract, but also patients with refractive problems like myopia (nearsightedness) hypermetropia (shortsightedness) and presbyopia (spectacle dependency in reading).
The aim of any IOL power calculation formula is to control the optical outcome of lens surgery. Because all presently available formulas use non-realistic models for the optics of the eye they require a number of corrective terms to be calculated in retrospect from observed data in order to work accurately. Examples of these ‘fudge’ factors include the ‘A-constant’ (SRK-formula), ‘Surgical Factor’ (Holladay) or ‘effective ELP or ACD’ (Hoffer or Binkhorst fomula). The effect of these ‘fudge’ factors is to correct for any off-set errors arising in the formula by applying an average corrective term making the predictions accurate in the average case.
When trying to predict the outcome of IOL surgery an accurate model of the eye is crucial. But no matter how precise and detailed the model of the eye ends up being, the position of the IOL in the eye cannot be measured before surgery and therefore has to be estimated. And without an exact knowledge of the final postoperative position of the IOL the postoperative refraction of the eye cannot be predicted substantially detailed to provide uncorrected vision for any patient who undergoes IOL surgery.
In the growing field of refractive lens surgery where patient demand and expectations are increasing, the surgeon may sometimes be faced with the problem that the patient ended up with an unexpected refractive result. Most of the times a review of the preoperative measurements may identify a measurement error as the cause of this ‘refractive surprise’. In other cases the reason for the wrong IOL power may not be evident, and the possibility of a formula error or a mislabelled IOL should be considered.